The present invention relates to a method for preparing synthetic powders based upon polyvinyl chloride or vinyl chloride copolymers by spray drying polymer dispersions through single material nozzles.
The state of the art of vinyl chloride polymer and copolymer emulsions useful in the present invention may be ascertained by reference to U.S. Pat. Nos. 2,140,048; 3,627,717; and 3,732,178, and the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Edition, Vol. 21, (1970), pages 369-412, particularly pages 373-374 where Emulsion Polymerization is disclosed, and pages 403-404 where Dispersion Resins which include plastisols and plastigels are disclosed. Kirk-Othmer states that plastigels are plastisols having high viscosities and yield values and that putty-like paste may be prepared therefrom. The state of the art of spray drying, pneumatic atomizers and polyvinyl chloride polymer and copolymer powders produced therewith may be ascertained by reference to U.S. Pat. No. 3,805,869 and Kirk-Othmer, Vol. 7 (1965), pages 360-368, under the section entitled "Dispersion Type Direct Dryers", particularly pages 362-364 wherein spray dryers and pneumatic atomizers are disclosed. U.S. Pat. No. 3,805,869 discloses the preparation of uniform, finely divided powdery vinyl chloride polymer particles containing less than 2 weight percent particles having a diameter greater than 40 microns obtained by spray drying an aqueous dispersion of the polymer and a carrier gas through a two-fluid atomizer.
As known to the art, plastisols i.e., dispersions of synthetic or plastic powders in softening agents, are prepared from these synthetic powders, as disclosed in Kainer's monograph, "Polyvinylchlorid und Vinylchlorid-Mischpolymerisate", Springer, 1965, page 332 ff. The industrially preferred method for making synthetic powders suitable for plastisol preparation is spray drying of polymer dispersions.
When such a process is used, the polymer dispersion is sprayed into a flow of hot air by means of an atomization instrument. The dried product is obtained at the lower end of the tower-like drying chamber via a bag filter or similar dust removing facility, as disclosed in Ullmann's "Encyklopaedie der Technischen Chemie", 1951, first volume, page 602 ff. Mainly the binary material nozzle is used in practice for drying polymer dispersions. The single material nozzle (liquid pressure nozzle) however is advantageous as regards energy consumption. Nevertheless, because it is very prone to clogging, it has not been widely used for the spray drying of synthetic powders. It is self-evident that ever recurring clogging of the nozzle channels causes unreliable operation and frequent shut-downs, and considerable cost in cleaning even if the nozzle can be easily taken apart.
The single material nozzle has not been used so far especially as regards the preparation of synthetic powders suitable for plastisol formation, because comparatively narrow nozzle cross sections are used which, therefore, tend to clog appreciably. These narrow nozzles are used in order to achieve the required fineness of particles.